1. Field of the Invention
The present invention relates to a telescopic antenna for use in vehicles and more particularly to a telescopic automobile antenna in which a rod-form antenna element can be removed from outside of the vehicle.
2. Prior Art
Telescopic antennas used in vehicles generally have the following type of structure: A telescopic rod-form antenna element made up of a plurality of conductive rods of different diameters is housed in a housing tube mounted on a vehicle body wall so that the antenna element can be inserted into and removed from the housing tube. The conductive rods of the antenna element are slidable relative to each other. The housing tube has a stopper which prevents the rod-form antenna element from slipping out of the housing tube. In other words, when the rod-form antenna element reaches its extension limit, the stopper prevents further extension of the antenna so that the antenna element does not completely come out of the housing tube.
In this type of telescopic antenna, the rod-form antenna element and the housing tube are formed into a single integral unit; i.e., such an antenna is not constructed so that the rod-form antenna element alone may be taken out of the housing tube from outside of the vehicle body. Accordingly, when the rod-form antenna element breaks or suffers problems and needs to be replaced, the entire antenna housing tube (including the antenna element) must be removed from the vehicle body, exchanged for a new unit, and then installed from the inside of the vehicle body. Thus, replacement of the rod-form antenna element is complex and difficult, and quick and reliable antenna repair work becomes impossible.
In order to avoid such problems, an antenna in which the rod-form antenna element alone can be simply removed from outside of a vehicle has been developed. In this antenna, a rod-form antenna element which has a locking projection at the lower end is inserted into a housing tube (which is mounted to the vehicle body wall) through an antenna insertion hole which opens in the upper portion of the housing tube. The antenna element is inserted from outside of the vehicle body, and a fastening element which is a tubular bolt is then screwed into the insertion hole so that the fastening element prevents the locking projection from passing through the antenna insertion hole to outside of the vehicle body. Thus, the rod-form antenna element is prevented from falling out of the vehicle body when the antenna is in use.
According to this type of antenna, the rod-form antenna element alone can be replaced from outside of the vehicle body by unscrewing and screwing the fastening element. Therefore, it is advantageous in that the antenna element can be more efficiently replaced compared to other types of conventional antennas.
However, there are problems which still remain unsolved. A cap which acts both as an ornament and as a protective device is attached at the tip (top end) of the smallest diameter conductive rods (hereinafter called "smallest-diameter rod") which comprises the antenna element. The diameter of this cap is usually equal to or smaller than the diameter of the largest diameter conductive rod (called "largest-diameter rod").
The reason for the particular diameter size of the cap is as follows: When the rod-form antenna is removed from the housing tube to replace it with a new one, the fastening element which is fitted over the rod-form antenna element is also moved. Ordinarily, this fastening element does not suffer any damage and can be reused. Accordingly, it is not economical to discard the fastening element with the rod-form antenna element. In order to reuse this fastening element, it must be removed from the rod-form antenna element either from the base end of the antenna element or from the tip end thereof. However, since the locking projection is fastened to the base end of the antenna element, the fastening element is not removed from the base end. Thus, the diameter of the cap is equal or smaller than the diameter of the largest-diameter rod so that the fastening element can be removed from the tip end of the antenna element.
However, if the diameter of the cap is equal or smaller than the diameter of the largest-diameter rod, some inconveniences arise. That is, when the antenna is not in use and the rod-form antenna element is completely retracted inside the housing tube, a small gap remains between the edge of the antenna insertion opening of the housing tube and the largest-diameter rod. This gap can act as a point of ingress for rain water, dust, etc. Accordingly, the gap should be covered with a cap. However, if the cap is larger in diameter than the largest-diameter rod to cover the gap, it becomes impossible to remove the fastening element from the rod-form antenna element via the tip end thereof so as to replace it. Thus, in this case, reuse of the fastening element is not possible.